and rebounds that have been hard to uncover
because of gaps in the fossil record. An under-
standing of these processes could reveal paral-
lels to the planet’s current loss of biodiversity.Patchy record
Most organisms in Earth’s history didn’t leave
fossils, and scientists have identified only a
tiny fraction of those that did. As a result, it
can be hard to tell whether changes in the
fossil record mark real shifts, such as mass
extinctions, or are simply caused by a lack of
fossil finds.
In the 1960s, palaeontologists began
analysing the fossil record systematically,
revealing multiple mass extinctions and
periods during which life flourished. But
these and later efforts could usually pinpoint
biodiversity changes to within only about ten
million years, because fossils were lumped into
relatively long geological periods and analysed
en masse.
To improve on this, a team led by palaeon-
tologist Jun-xuan Fan at Nanjing University in
China created and analysed a database of fossil
marine invertebrates that were found in more
than 3,000 layers of rock, mostly from China
but representing geology around the planet
during the early Palaeozoic. The group then
used software to measure when individual
species had emerged and gone extinct.
The program took advantage of the fact
that species were usually found in multiple
rock formations — each spanning hundreds
of thousands to millions of years — and used
this information to place upper and lower lim-
its on the period in which the species actually
existed. The effort revealed for how long, and
in what order, all 11,000 species had existed. It
took the supercomputer around seven million
processor hours.Trilobites disappeared from the fossil record during a mass extinction 252 million years ago.Using this approach, the team was able to
learn extra details about events such as the
end-Permian extinction, and the Cambrian
explosion around 540 million years ago. The
analysis showed, for instance, that species
diversity declined in the 80,000 years leading
up to the end-Permian mass extinction, which
itself occurred over about 60,000 years.The findings also cast doubt on the exist-
ence of a smaller-scale die-off known as the
end-Guadalupian extinction, which is thought
to have wiped out many marine species around
260 million years ago. That was the biggest
surprise, says Mike Benton, a palaeontologist
at the University of Bristol, UK, who has docu-
mented changes in vertebrate diversity during
that period. The study, he adds, “represents a
pretty amazing big-data endeavour”.
Benton hopes to see the effort extended to
later periods — particularly the past 100 million
years. Palaeontologists disagree over whether
an apparent increase in animal diversity in this
period is the result of sampling bias.
Norman MacLeod, a palaeontologist at
the University of Nanjing and a co-author of
the study, says the team’s work might help to
reveal the underlying causes of changes in
biodiversity, by charting ups and downs on a
timescale that can be matched with environ-
mental and climatic shifts.
Wagner adds that the team’s approach will
be most valuable in uncovering — and explain-
ing — smaller-scale extinctions, not dissimilar
to those occurring today. Such extinctions
could turn out to be “a bad 100,000 years, or
a bad week” for some groups of organisms but
not others, he says. “When you get this reso-
lution, it starts opening the doors to actually
testing what the smaller-turnover events
might be like.”By Nidhi SubbaramanS
cientists can now create clumps of cells
that resemble human embryos, raising
hopes that they could study the elusive
first stages of human development
while avoiding the ethical concerns
that make it difficult to study actual human
embryos. But as these embryo models — in
which human stem cells are transformed into
embryo-like structures whose growth mirrors
stages of embryonic development — grow in
popularity, US researchers say that they are
finding it increasingly difficult to obtain fed-
eral funding for such work.
The US National Institutes of Health (NIH)
in Bethesda, Maryland, has funded and still
does fund work on embryo-like structures.A spokesperson told Nature that the agency
considers grant applications involving mod-
els that “could be considered an organism” on
a “case-by-case basis”, and cited a provision
of federal law known as the Dickey–Wicker
Amendment, which bars the government
from funding research that creates or destroys
human embryos.
But the ban, which dates back to 1996,
was put in place before the advent of tech-
niques that produce embryo-like structures
from stem cells. Scientists working on such
research say that they need clearer guidance
on what is eligible for federal funding. “The
writing on the wall is that synthetic embryos
are out of bounds with the NIH. The next step
in the science is not allowed,” says Eric Siggia, a
physicist who studies developmental systemsBiologists say they need clearer guidelines
on funding rules for this nascent field.STUDIES OF EMBRYO-LIKE
STRUCTURES STRUGGLE
TO WIN US GRANTS
SHUTTERSTOCKNature | Vol 577 | 23 January 2020 | 459
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